1. Field of the Invention
The present invention relates to an electrical safety compliance analyzer, and in particular to an electrical safety compliance analyzer capable of performing multiple safety compliance tests, including AC and DC dielectric voltage withstand tests, an insulation resistance test, and a ground circuit test.
2. Description of Related Art
A. Types of Electrical Safety Compliance Tests
In a ground referenced power supply circuit such as is commonly used in the United States, leakage current due to faulty insulation or improper design will flow from the power circuit of an electrical circuit to ground via the lowest impedance path, injuring anyone in that path. To prevent such injuries, electrical insulation in the product must be free from defects, and the ground circuit must be functioning properly, i.e., the ground circuit of the electrical product must be the lowest impedance path to ground.
A number of tests are currently used to determine insulation quality in order meet safety regulations for the products tested, and prevent injuries to users. Each test has advantages and disadvantages which make it particularly suitable for use in connection with particular products, but not others, and thus it is generally necessary for a manufacturer or testing agency to be able to perform a number of different types of safety compliance tests in order to meet the different testing requirements for different products.
Insulation quality has traditionally been measured using a procedure referred to as the insulation resistance test. The insulation resistance test measures the quality of insulation in a product or component by applying a voltage corresponding to the rated voltage of the product and determining the total resistance of all elements situated in the path of current between the test points to which the voltage is applied. This test is simple and accurate, but cannot detect certain causes of incipient failure such as pinholes, weak insulating materials, inadequate spacing of components, and pinched insulation.
Another way to test the quality of insulation is to measure leakage current to ground at operating voltages of an electrical product. This is generally accomplished by connecting an ammeter between an isolated part of the product being tested and its neutral conductor, and measuring any current sensed by the ammeter when the operating voltage is applied.
A further variation of the conventional leakage current is the dielectric withstand test, also known as the hipot test, which also measures leakage current, but involves applying a higher voltage to the two points, so as to stress the insulation far beyond what it will encounter during normal use, and measuring stray current through the insulation. For most applications, this test requires application of an AC or DC test voltage of at least twice the normal operating voltage of the product being tested, plus 1,000 volts.
AC hipot testing is more generally accepted by safety compliance agencies than DC hipot testing because AC testing stresses insulation in a manner that more closely simulates stresses the product will see in normal use. Since AC testing cannot charge a capacitive load, the current reading remains consistent from initial application of the voltage to the end of the test, and there is no need to gradually bring up the voltage unless the product is sensitive to a sudden application of voltage. In addition, unlike DC hipot testing, AC hipot testing does not necessitate discharge of the electrical product being tested after the test has been completed. However, disadvantages of AC hipot testing include the effect on test results of reactive currents due to capacitances in the product, which are difficult to distinguish from leakage currents, and the greater power required in the presence of both reactive and leakage currents.
DC hipot testing, on the other hand, has the advantage of requiring less power than AC hipot testing because reactive currents are not present and also because the charging current only needs to be applied long enough to charge the item under test. On the other hand, DC hipot testing has the disadvantage that the voltage must be ramped slowly to the full test voltage so as to prevent the charging current from giving false indications of failure. In addition, DC hipot testing is disadvantageous because it requires that the product being tested be discharged after testing, and because the test can only charge an item to one polarity, rather than alternating polarities as in AC testing.
Ground circuit testing has generally involved performing a low current continuity test on the ground circuit of the product being tested, and determining whether a threshold amount of current, for example current sufficient to light an indicator lamp, passes through the ground circuit. While continuity tests utilize simple low power circuitry and reliably indicate the presence or absence of a ground connection, however, they do not test the integrity of the connection, and thus many applications call for stress tests, referred to herein as ground bond tests, which measure the actual impedance of the circuit at relatively high currents. Current ground bond testing standards require test currents of between 10 and 30 amps at test voltages of less than 12 volts. Because of the small impedance of a ground circuit, the ground bond test requires that the kelvin method or an offset be used to account for the resistance of test leads or other connections which are not part of the ground circuit being tested.
B. Conventional Test Instruments
The present invention concerns an instrument that adds insulation resistance and ground circuit test capabilities to an AC/DC hipot test instrument, using common power and measurement circuitry to the extent possible in order to eliminate redundant circuit elements which would increase the cost and weight of the instrument. While all of the above-described tests are well known to those skilled in the art of safety compliance testing, and most safety compliance regulations specify the types of tests that must be performed on a particular product, the present invention is unique in combining AC and DC hipot test instrumentation with instrumentation capable of carrying out insulation resistance and ground circuit tests, including in one preferred embodiment of the invention the capability of carrying out ground bond tests.
It has of course been previously proposed to provide instruments capable of carrying out a variety of different electrical safety compliance tests. However, while instruments capable of supplying different output voltages for insulation resistance and leakage tests, or of combining continuity and leakage current test capabilities are described in a number of prior publications and are currently commercially available, the unique nature of dielectric withstand or hipot tests has heretofore prevented effective or commercially viable integration of hipot test equipment with instruments also designed to carry out insulation resistance and ground circuit impedance tests.
Examples of prior instruments capable of performing leakage current and ground circuit tests, but only at line or operating voltages, rather than voltages sufficient to stress the insulation of the products being tested, are found in U.S. Pat. Nos. 2,968,763, 3,170,115, 3,643,157, 3,704,411, 3,878,458, 3,963,981, 4,578,637. While some of the insulation resistance testers use a voltage higher than the 110/220 volt line voltage, none provides a voltage high enough to accomplish dielectric withstand testing, and none provides more than three different types of tests. An example of an instrument which provides three different types of tests is disclosed in U.S. Pat. No. 3,141,128, but none of the three tests provided by the disclosed instrument meets the hipot requirement of applying a voltage of at least one thousand volts plus twice the operating voltage of the product being tested, the three tests instead being a low voltage ground impedance test, a 750 volt DC insulation resistance test, and a test for leakage current at the 120 volt AC operating current.
In contrast to the combined ground and insulation test instruments described in the above-cited patents, a conventional DC hipot leakage test instrument such as the one disclosed in U.S. Pat. No. 5,514,967 does not provide any sort of insulation resistance or ground circuit impedance testing. Even the state-of-the-art power circuitry described in U.S. Pat. No. 5,548,501, aspects of which are used in the power circuitry of the preferred embodiment of the present invention, and which provides for both AC and DC high voltage outputs, fails to include any provision for combining the high voltage power supply with insulation and ground bond test circuitry.
One of the principal reasons why combinations of low voltage ground circuit and leakage resistance test equipment with hipot test equipment has not previously been achieved is the different set-up and usage protocols required for the instruments. Hipot test equipment presents significant risks to the operator because of the high voltages involved, particularly during setting of the voltage and current trip parameters which, in conventional hipot instruments, is carried out at full power, and during calibration of the equipment which, in conventional instruments, requires difficult and time consuming manual adjustment of potentiometers. In an instrument that simply combined prior instruments, the greater number of outputs and adjustments would unacceptably increase the chances of operator error and risk of injury, particularly if the operator were to mistakenly set a high voltage parameter for a low voltage test.
Such safety and ease-of-use concerns, combined with difficulties in combining such disparate instruments in an efficient manner, have thus far precluded any attempts to combine AC and/or DC hipot test instruments with insulation resistance and ground circuit test equipment. As a result, the present invention is the first to provide the capability of performing all four types of tests using a single instrument.